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Roof Catchment Rainwater Harvesting Techniques

Show cases by Wilfred Muriithi

1. The household approach

In the household approach, water is harvested from the roof of the household and stored in tanks made of different

materials, namely Ferro-cement, bricks and or polyethylene.

IGAD BMP/ICRAF tanks will have a useable volume of 25 m³. For convenience in acquiring the plastic lining

material; a piece of HDPE dam liner 8m wide x 10 meters long will give 10m x 8 m liner which will be sufficient to

line a 25 m3 pond. A roof 90 m2 in an 800mm annual rainfall zone will suffice. The tank will be suitable for a family

of 5 (a 5 m x 10 m vegetable garden and 200 tree seedling tree nursery).

Criteria used in selection of technologies.

(a) Domestic use – Factors considered

- Catchment type and size

- Cost of the system (used)

- Family size

- Length of drought period

- Alternative water sources

(b) Crop production (factors)

- Rainfall amount, intensities, evapotranspiration rates (water balance accounting)

- Soil characteristics (infiltration rates, holding capacity, fertility, depth)

- Crop characteristics (Wba )= H2O req. length of growing period, resistance to water logging

- Hydrology

- Social economic factors – Family size, labour, people’s priorities, cost of materials, land tenure,

regulations governing water resource use.

(c) Effective water Harvesting required community (group members) participation;

- Sensitivity to the people’s needs

- Indigenous knowledge, local expertise, farmer’s initiatives and adaptability

- Full participation at all levels – gender sensitive

- Consideration of prevailing farming systems, national policies.

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Design/construction for storage tanks – consideration done to

1. Average annual rainfall

2. Size of the catchments

3. Water demand

(a) Annual water harvesting potential

QR = A RC

A = Area of the catchment

B = Average annual rainfall

C = Runoff coefficient (roof = 0.85)

(b) The tank is designed for use during the dry period e.g. for a family e.g.

Qf = # persons x daily requirement x # days

As a safety factor, the tank should be 20% larger than required.

1. Domestic water RWH for a family of 5 persons

Standard tank size Capacity of tank = 9.8m3

Family demand

Qf = #persons x daily demand x length of dry period

= at 20 litres/person/day the tank will last

9800 litres = 98 days (3 months)

5 x 20

Vegetable garden demand (3 months) 50 m2 @10 litres/m2/week x 13 weeks=6.5m3 9.8+6.5=16.3 m3 demand for human and vegetable garden and surplus for loses Design 1 25m3 Plastic lined trapezoidal tank 1m underground and 1m above ground Dimensions-Top=6m x 3 m, Bottom=4m x 2 m, Depth=2m Iron roofed (gabble) Above ground section constructed with stone, blocks or reinforced marram bricks

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Construction method for a 25m3 plastic lined tank

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4 5 6

7 8 1. Set foundation 3m x 6m 2. Make a foundation trench 30cm wide and 15 cm deep 3. Construct a wall,1 m high above ground 4. Excavate a whole 2m x 5m x 1 m deep inside the construction 5. Slope the sides to create a trapezoidal tank 6. Make a trench at the bottom from one edge to facilitate installation of a drain pipe and install the dam liner 7. Roof the tank to control encroachment and subsequent interference from external forces 8. Connect the tank to the catchment

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Annex i Trained technicians drawn from groups within Witu Item Name Affiliation Contact

ID Telephone 1. Onesmus Songoro LCT 26860304 None 2. Kahindi Karisa Mulumba N. k.Bee Keepers 4982525 0728489557 3. Josphat Mbitha LCT 31325541 0702269962 4. Caleb Ogweri N.KBee Keepers 25638136 0707583171 5. Joseph Makori Back to Eden 10468952 0710560323 6. Joseph Ngumbao Back to Eden 2237322 0716664253 7. Salim Kofa LCT 30031995 0716652518 8. Kalama Nzaro New Kumwkucha 11876442 0723903385 9. Silvesta Jefwa New Kumekucha 13375076 0787655024 10. Benard Mururu Witu Bee Keepers 24769878 None 11. Paul Mwangi Soroko Hort 11141803 0716650604 12. Mohamed Simba LCT 24666292 0728753759 13. Dismus Songoro LCT 32968910 None 14. Imanyara N’kanata Witu bee keeper 3747265 0729246735 15. Misheck Kirinya Witubee Keeper 4347352 0719791671 16. Benard Kitheka Back to Eden 4834544 0700592824

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Annex ii

Materials required (BOQ)

Item Material/inputs Unit No Unit cost Total cost

A

Storage system

1 Plastic lining 8x10m piece 1 300 300

Roofing sheet Long 6 piece 15 450 6750

2 Building blocks 6x9’’piece 180 40 7200

3 Sand tone 1.5 1000 1500

4 cement 50kg bags 5 650 3250

5 Ballast(graviere) tone 1.5 1000 1500

6 Timber Running m 64 15 960

7 Building nails ,4&5’’ Kg 5 100 500

8 Roofing nails kg 1 150 150

9 Labour

(skilled/unskilled)

Man days 50 500 25000

47110

B

Conveyance System

32mm PE pipe 100m roll 1 7000 7000

pump piece 1 15000 15000

Micro- sprinklers pieces 5 250 1250

Extension cable

(electrical)

30m piece 1 200 200

gutters 4 250 1000

Down pipes 1 750 750

25200

Gross total 52310

Artisan qualification The trained artisans were able to construct the plastic lined tanks for their groups; Kakate team constructed a 33m3 tank,Tangeni 2 x 25 m3 tanks and Soroko 1 x 10 m3 tank.

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Design 2 Runoff pond on Large Roof Catchment System Witu secondary School 190 m3 plastic lined pond Dimensions Top 9 x 13m,bottom 9.5 x 9m,depth 2.7m(water depth 2.5m,free board 0.2 m expected volume 190m3) Roof catchment =41 m x 11 m Available area to be irrigated a) 10 x23m b) 10 m 14 m =370m2 Pond irrigation potential POND USES

1. Training 2. Vegetable production 3. Fish farming 4. Raising tree seedlings 5. Sustaining newly planted seedlings 6. Students’ project

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1. Signage at Witu Secondary School

2. 190m3 Plastic lined runoff pond at Witu

Secondary School.

3. The pond seen covered with 80% shade

net to keep off mosquitos, insect

pest/roddents, evaporation and

equipped is also with a rope/washer

pump to ease water utilization